1. Field of Invention
The present invention relates to improved light-control devices of the active type (the term "light" being used herein to refer in the generic sense to infrared, ultraviolet and microwave radiation in the neighboring portions of the electromagnetic spectrum, as well as to visible light). More particularly the invention relates to devices in which an element, comprising field-orientable particles supported within an operative area by an elastomeric medium, is field activated and deactivated to selectively control light, incident on the operative area of the element.
2. Description of Prior Art
Various elements and apparatus have been proposed in the prior art for selectively controlling incident light (e.g., regulating its transmission, reflection, diffraction or absorption) in response to the selective application of an electric or magnetic field to the element. A large number of the apparatus and elements of this kind can be grouped as being either active (i.e., primarily intended for rapid and recurring changes in light control effect) or inactive (i.e., primarily intended for a relatively permanent light control effect).
Typical prior art systems in the inactive group include devices or elements in which a surface deformation pattern is created on a film by heating during application of a corresponding electric field pattern (e.g., an electrostatic image) to the film. After such film deformation, visible light directed towards the film is affected in a manner creating an observable image corresponding to the field pattern. In a similar inactive system, electric-field-orientable, light-control particles are dispersed in heat softenable support and oriented, in an imagewise pattern, by application of a corresponding electric field pattern while heating.
The present invention has certain similarities to the aforedescribed inactive devices and elements, but relates more closely to apparatus and elements of the type generally categorized as active, wherein one particularly desirable attribute is a capability for rapid changeover from one light-controlling format to another.
Active control of light in correspondence with applied electric field, has been the subject of a number of different approaches and utilized in a variety of different environments. Common applications are for use as an electro-to-optical image converter and as an optical-image intensifier.
U.S. Pat. No. 2,290,581 discloses an early electro-to-optical converter in which graphite or aluminum particles suspended in a liquid medium were selectively aligned by a scanning electron gun to modulate the passage of light and provide a viewable image. Similar light control devices, using dipole particles in liquid suspension and orienting electric fields, are disclosed in U.S. Pat. Nos. 3,257,903; 3,512,876 and 3,527,525.
Another approach, disclosed in U.S. Pat. No. 2,457,981, utilizes an array of minute reflective fibers which are momentarily electrostatically charged, point by point with an electron gun, in accordance with a desired image pattern. Light directed onto the array is controllably reflected in accordance with the image pattern due to imagewise flexure of fibers, caused by temporary electrostatic charges from the scanning electron beam.
U.S. Pat. No. 3,100,817 discloses the use of a reflectively coated "electro-elastic" membrane, which changes in thickness (and therefore its optical reflective characteristic) under the influence of an electric field, to convert electrical patterns to viewable ones.
U.S. Pat. No. 3,592,527 discloses the use of an element, comprising a liquid crystal-photoconductor laminate, to intensify light images. This element utilizes the clear-to-diffuse change occurring in liquid crystal under imagewise electric patterns on the photoconductor.
The prior-art approaches, to some extent at least, all suffer a high cost of fabrication, a fragile and cumbersome packaging and chemical instability, thus a high sensitivity to environmental changes, e.g., temperature change. Special optics and limited viewing angles are also common disadvantages of prior art systems. Certain systems, e.g., the flexed fiber system, are not adapted to receive and hold the image signals in parallel, i.e., simultaneously, and rely on a continuous scan which is itself costly and limiting as to field of application.
The dipole particle-liquid suspension systems of the type mentioned above have apparently been developed to a stage of commercial utility; however, one detractive feature of such systems is a slow recovery time, caused by reliance on Brownian movement to return the suspended dipole particles to the unoriented condition.
The patents mentioned above describe only some of the applications in which active light-control devices can be useful; and in view of the deficiencies of prior art devices of this type, it can be seen that a need is present for new and improved light-control devices of the active type described.